Oxene intermediates

The high degree of electrophilicity of the oxene intermediate would facilitate hydrogen-atom abstraction from substrates such as the methyl group of N,N-dimethylaniline to generate a "crypto-hydroxyl metal center able to undergo the well-known "oxygen rebound mechanism.22... 

The results of Table 4-6 and the close parallels of the product profiles to those for the FeJI(PA)2/HOOH/(py/HOAc) system36 prompt the conclusion that the combination of CoII(bpy) (18) and HOOH results in the initial formation of an oxene intermediate [(bpy)2 Co O-, 19], which (in MeCN/py) rapidly reacts with a second HOOH to give a dioxygenase reactive intermediate [(bpy)2 o" (O2), 20] (Scheme 4-6). 

Scheme V. Proposed mechanism for CCP compound I formation, (a) The native enzymes (b) the activated complex with the distal histidine operating as an acid-base catalyst and the active site arginine stabilizing a developing negative charge on RO-OFe (c) the hypothetical oxene intermediate (d) Compound I after the intramolecular electron rearrangement of (c) to give Fe(IV) and a hee radical, X.

Figure 21.2. Activation of O2 or H2O2 by POMs, through the formation of oxene intermediates (upper right) or metal-peroxo intermediates (lower right).

Such a radical-pair mechanism avoids the need to invoke a oxene intermediate during the oxygen transfer by the flavoenzyme, and at the same time releases the ring strain of the oxaziridine species. 

The same aldehyde is formed in the oxygenation of quadricyclane catalyzed by cytochrome P-450 [88]. The mechanism of the cytochrome P-450-catalyzed reaction has been reported as follows the oxygenation is initiated by one electron transfer from quadricyclane to an electron deficient iron-oxene intermediate, which is followed by insertion of an oxygen atom into a C-C bond rather than a C-H bond. The electron transfer from substrate to the iron-oxene intermediate may be reasonable because of a high ionization potential of quadricyclane. Since the diagnostic aldehyde is a sole product in both sMMO and cytochrome P-450 systems, it is likely that the same reaction mechanism is applicable to these two enzymatic reactions. A plausible mechanism for the sMMO-catalyzed oxygenation of quadricyclane is shown in Scheme 4. 

To this end, cytochrome P-450 may be considered as nature s equivalent of Fenton s reagent in the enzyme system, highly reactive intermediates such as hydroxyl radicals and oxenes appear to be controlled by their interaction with hemoprotein. 

FIGURE 4.78 Mechanistic pathways for aromatic hydroxylation by concerted addition of oxene, pathway 1, or by stepwise addition of oxene, pathway 2. Pathways 2, 3, and 4 describe the formation of phenol that bypasses the arene oxide intermediate. 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

The ferric peroxide intermediate could undergo either heterolytic or homolytic cleavage to give either a ferryl or diferryl oxene. In the path shown in Fig. 14, the organic radical is generated by reaction with the Fe center after heterolytic 0—0 bond cleavage. For RNRB2 this is the... 

The pL-independent IEs have therefore been interpreted580 as indicating that DOD shifts the acid-base equilibrium from (Fem—OO-) to the protonated intermediate (Fenl—OOH) and increases in that way the rate of synthesis of products formed via oxene chemistry (inverse SDIE) and decreases the rate of products formed through the peroxide chemistry... 

Figure 6.2 The mechanism of cytochrome-c-peroxidase complex formation, (a) Native enzyme, (b) Activated complex with the acid-base catalytic function of distal histidine (His) and stabilization of negative charge by arginine (Arg) residue of the active site, (c) Hypothetic intermediate oxene complex, (d) Complex I after intramolecular electron regrouping of oxene complex with Fe4+ and free radical X fragment formation.

The trioxolane was also shown to effect epoxidation of olefins86). This was differentiated from the peracid pathway by generating the intermediate at — 100 °C. Warming this system to — 50 °C in the presence of olefins resulted in the formation of epoxides. The corresponding experiment with peroxyacetic acid at — SO °C showed no epoxidation of the olefins. The trioxolane was also shown to effect hydroxylation of tert-butylphenol to tert-butylcatechol, in agreement with an oxene transfer mechanism. 

Sugimoto, H., Tung, H.-C., Sawyer, D. T. (1988). Formation, characterization, and reactivity of the oxene adduct of [Tetrakis(2,6-dichlorophenyl)porphinato]iron(III) perchlorate in acetonitrile. Model for the reactive intermediate of cytochrome P-450,... 

Hamilton suggested that the model systems (at least some of them) interact with the substrates via a so-called oxenoid mechanism similar to that of monooxygenase functioning. Since the reaction, in many aspects, parallels the processes with carbenes (addition to the multiple bond, insertion into the C-H bond), in the oxenoid mechanism an oxygen atom (oxene) inserts into the C-H bond without an intermediate formation of free radicals ... 

Most of investigators postulate the pentavalent iron oxene complex, P450-Fe =0, as the direct oxidant of a substrate by analogy with the reactive intermediates of the catalytic cycle of peroxidase, which have the oxoferryl group, Fe =0, in their active center. However, the Mossbauer and Raman spec-... 

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